Device for measuring porefluid pressures



March 26, 1968 DEVICE FOR MEASURING FORE-FLUID PRESSURES Filed June 1, 1966 J. R. LEFELHOCZ ET AL INVENTORS JOSEPH R. LEFELHOCZ ALLAN D. BERGMANN CLETUS E. PEELER, JR.

United States Patent ()fifice 3,374,664 Patented Mar. 26, 1968 3,374,664 DEVICE FOR MEASURING PORE- FLUID PRESSURES Joseph R. Lefe'hocz, Cletus E. Peeler, Jr., and Allan D. Bergmann, Painesville, Ohio, assignors to Diamond Shamrock Corporation, a corporation of Delaware Filed June 1, 1966, Ser. No. 554,416 3 Claims. (Cl. 73-73) Thi invention relates to a device for measuring porefluid pressures in earthen formations and more particularly relates to an improvement in porous tube piezometers whereby they may be used in earthen formations having a wide variety of soil particle sizes.

It is known to use porous tube piezometers for measuring pore-fluid pressures in earthen formations such as dams, areas surrounding building foundations and the like where it is desirable to know the degree of underground water flow or pressure. These devices generally consist of a length of porous, hollow, aluminum oxide or silicon carbide tube plugged at one end and having a riser tube inserted in its other end and extending to the surface of the earth. In installation, the piezometer is placed in a hole drilled to a predetermined and desired depth. Sand or gravel is then placed around and above the tube followed usually by a chemical grouting composition to secure said riser tube. After installation and when equilibrium has been reached with the ground water present, an elecrical probe may be lowered into the riser tube whereby, upon contacting water in the tube, a resistance change is noted and by referring to the calibrated lead wire attached to the probe, the height of water in the tube and hence the pore-fluid pressure may be known. The use and design of these piezometers is InOre completely described on pages 658-672 of the Earth Manual of the United States Department of the Interior-Bureau of Reclamation, first edition revised, 1963.

While this type of porous tube piezometer has met with some success, its chief disadvantage has been that in clay or other earths having soils of extremely small particle size, there is a tendency for silt to penetrate the porous tube and either fall to the bottom of the piezometer or clog the pores; in either case reducing the effective area for response tofluid pressure changes. While some control of this may be obtained by installation of the device in a carefully laid sand bed this is not entirely effective, allowing fines still to penetrate to the tube itself along with the water. A further, and more effective control may be obtained by choosing the porosity of the piezometer to suit the general particle size of the soil in which it is to be used. Unfortunately this is still not a complete solution to the problem as these tubes are generally available in only three porosities, coarse, medium and fine. Obviously, soils having a much wider variety of particle size will be found in the field.

It is an obeject of this invention to provide a porous tube piezometer for use in determining pore-fluid pressures in earthen formations.

It is a further object of this invention to provide a modification in existing porous tube piezometers whereby they may be readily adapted to be used in earthen formations regardless of the existing soil particle size.

A still further object of this invention is to provide a modfied porous tube piezometer having a rapid response to pore-fluid pressures and an improved resistance to clogging by fine soil particles.

These and other objects of this invention will become apparent to those skilled in the art from the description that follows and by reference to the drawing which is attached hereto and incorporated as a part hereof.

In accordance with the above objects there is provided an improvement in porous tube piezometers consisting of a length of hollow, porous tubing having a stopper in its lower end and a riser tube extending from its upper end said improvement comprising providing the outer surface of said porous tube with a covering of a flexible, corrosion resistant, fluid-permeable screen having openings therein of a mesh size chosen to at least conform to the particle size of the soils surrounding the borehole in which the piezometer is to be installed.

The figure in the drawings is a vertical sectional view through a portion of the porous tube piezometer.

Referring now to the drawing a typical piezometer in accordance with this invention comprises a length of hollow, porous tubing 1 having a tapered stopper 3 in its lower end and a length of flexible, hollow tubing 5 inserted in and extending from its other end and having it entire surface covered with a single layer covering of flexible, corrosion resistant, fluid-permeable screen 7. Inserted in and extending from the flexible hollow tube 5 is a riser tube 9 held in place in tube 7 by a clamp 11.

The porous tubes useful in this invention may be of any material that may be fabricated into lengths of hollow tubing such as silicon carbide or aluminum oxide. Generally these tubes are commercially available in three grades of porosity; coarse, having an average pore size of 240 microns, medium, having an average pore size of 164 microns and fine, having an average pore size of microns. These tubes may conveniently be from 2 to 4 feet in length, have an outside diameter of about 1% inches and an inside diameter of about inch, although these dimensions should in no way be interpreted as being critical as they may be varied widely without affecting the operation of the piezometer. At present it is preferred to use a tube having the above described coarse designation since this grade of tube, because of its larger pore size, allows a more rapid response to fluctuating pore-fluid pressures.

As stated above the improvement embodied in the present invention consists in providing the above described porous tube with a covering of a flexible, corrosion resistant, fluid-permeable screen, said screen being selected to conform to the particle size of the soils existing in the formation in which the piezometer is to be installed. That is to say that Where soil particle sizes are known to be quite small, a fluid-permeable screen having a correspondingly small mesh size will be selected.

It has been found that, while other materials such as plastics may be used, a screen constructed of nylon is particularly useful in the practice of this invention. The main reason for the preference for nylon screen is to be found in the fact that it is commercially available in a wide variety of mesh sizes. By mesh sizes it is intended to refer to the pore size openings in the screen itself and more particularly to its corresponding rating in Standard U.S. Sieve sizes. This nylon screen is readily available in mesh sizes ranging from 6 (3,360 micron openings) to 400 (37 micron openings).

The method of attaching the fluid permeable screen to the outer surface of the porous tube is not critical. This attachment conveniently may be effected by cutting a portion of the screen in sucha size that it may be wrapped around the outside of said tube to form a single layer of screening plus a small overlap. By then applying a small amount of adhesive such as an epoxy-type glue between this overlap, an etficient and effective seal is provided and the screen will be held in position, with normal care, while installing the complete piezometer.

The advantages afforded by the practice of the present invention will be obvious to those skilled in the art.

In the past it has been possible only to choose the porosity of the porous tube from the three grades available, coarse, medium and fine. Obviously soil particle sizes vary over a much wider range than this and it is now possible, by choosing a fluid permeable screen having the proper pore, or mesh, size, to conform much more closely to existing soil conditions. Thus, it is possible to achieve an optimum balance between response to porefluid pressure variations and filtration of fine soil particles to prevent clogging of the pores and filling of the inside of the tube itself with very fine soil particles. It is now possible to use a porous tube having a coarse designation as to pore size and thereby receive the benefits of rapid response to pressure changes while still preventing clogging of the tube by choice of the proper porosity fluid-permeable screen.

Furthermore, the use of a fluid-permeable screen of nylon, by the very nature of its surface, provides an additional means to prevent the plugging of the pores of the piezometer.

In order that those skilled in the art may more readily understand the nature and use of the modified porous tube piezometers of this invention, the following description of atypical installation is alforded.

The piezometer itself is constructedstarting with a 28 inch length of medium porosity aluminum oxide tubing having an outside diameter of 1% inches and an inside diameter of Va inch. In one end of the tube is inserted a tapered rubber stopper to provide an effective bottom seal. Into the other end is inserted a 5-inch length of rubber tubing having an outside diameter of '78 inch and a /s inch thick wall. Into this rubber tubing is then inserted, in a length appropriate to reach the surface, a polyethylene riser tube having an inside diameter of /2 inch and a wall thickness of ,4 inch. A rubber tubing, or airplane clamp is used to secure the riser tube in the rubber tube. The outside surface of the porous tube is now fitted with a single layer cover-' ing of 325 mesh (pore size 44 microns) nylon screen cemented together and in place by a strip of epoxy glue. The piezometer is now ready for installation.

A 4-inch diameter hole is drilled to the depth at which it is desired to know the pore fluid pressure. The hole is then extended for an additional 3 feet, a well casing is inserted to the bottom and the hole is flushed clean with water. At this point the casing is slowly withdrawn for 3 feet while sand, saturated with water, is poured in to provide a porous bed on which the piezometer will rest.

The piezometer assembly is connected to a water supply tank and filled with water, then when subsequently lowering the piezometer into the casing and performing the subsequent back-filling operations, a small positive pressure is maintained on the supply tank to provide an outward flow from the piezometer tube and prevent its plugging during the remaining installation proceedings.

The piezometer is now lowered into the casing until its rests on the previously placed sand bed. The casing is now withdrawn in increments while adding saturated sand until the sand back fill surrounds the piezometer and extends to at least 12 inches above the top of same. The riser tube is secured in place by withdrawing the casing in increments while filling the drill-hole with a bentonite drilling mud worked to a putty-like consistency. With all of the casing removed a Zinch diameter pipe is placed in the top 2-3 feet of the hole and extending about 1 foot above ground to protect the riser tube which is now disconnected from the water supply tank. This protector pipe may then be fitted with a cap to prevent foreign matter from entering the riser tube when readings are not actually being taken.

After standing to allow the water in the piezometer tube to come to an equilibrium with the fluid pressure in the surrounding earth strata, measurements are taken, at desired time intervals, by lowering an electrical contact probe connected by means of an insulated lead wire to an ohmmeter. Upon contact of the probe with the water level in the tube, the operator notes a significant change in the resistance indicated by the ohmmeter. He then ceases lowering the probe and by reference to the lead wire, which has previously been marked ofi in suitable gradients, he obtains a direct reading of the water level in the tube and, hence, the pore-fluid pressure in the strata surrounding the porous tube piezometer.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes or alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. In an apparatus for determining pore-fluid pressures comprising a hollow, porous tube having a stopper in one end and a riser tube extending from its other end, the improvement which consists in providing said porous tube with a covering of a flexible, corrosion resistant, fluidpermeable screen.

2. An apparatus as in claim 1 wherein said hollow, porous tube is constructed of a material selected from the group consisting of silicon carbide and aluminum oxide and said fluid-permeable screen is constructed of nylon mesh.

3. An apparatus as in claim 1 wherein the size of the openings in said fluid-permeable screen are chosen so that they at least conform to the soil particle size in the earthen formation in which it is to be installed.

References Cited UNITED STATESPATENTS 3,091,115 5/1963 Roberts 7373 3,146,617 9/ 1964 Klein et a1 73-73 LOUIS R. PRINCE, Primary Examiner.

D. O. WOODI-EL, Assistant Examiner. 

1. IN AN APPARATUS FOR DETERMINING PORE-FLUID PRESSURES COMPRISING A HOLLOW, POROUS TUBE HAVING A STOPPER IN ONE END AND A RISER TUBE EXTENDING FROM ITS OTHER END, THE IMPROVEMENT WHICH CONSISTS IN PROVIDING SAID POROUS TUBE WITH A COVERING OF A FLEXIBLE, CORROSION RESISTANT, FLUIDPERMEABLE SCREEN. 